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APPENDIX II. 
with tlie extensive mud-flat -which extends at a depth of a 
few feet below the surface northward of the Keys. Where 
there is a larger accumulation of material than usual on 
the submarine plateau, so as to bring its surface within the 
depth at which corals can flourish, a reef begins to form ; 
that is the history of the Tortugas. West of it, an in- 
cipient reef may he found now in process of formation, 
east of it all the reefs in their turn have had a like origin. 
Then the deposition of silt produces mud-flats, and material 
accumulates, till at last the channels are closed and the 
whole is added to the land. From Everglades to Cape 
Sable the work may be seen completed ; on the eastern 
coast, and beyond the latter place to Marquesas Key, it 
occurs in its various stages, until at last it is shown in its 
beginning. The backbone, however, of the Florida penin- 
sula is ascribed to a fold in the earth’s crust in an earlier 
geological period. As a secondary result of this a great 
submarine plateau was formed directly in the track of the 
Gulf Stream, which has since been gradually built up by 
the accumulation of marine organisms of various kinds. 
The area within the 100-fathom line on the west coast of 
the great Florida plateau is extraordinarily rich in organic 
life ; large fragments of the modern limestone were often 
brought up in trawl or dredge, consisting of the solid parts 
of the very species wdiich now live on the top of this 
recent limestone. West of the western shore-line Florida 
now stretches out as an immense submarine plateau, 
forming a huge tongue, coated or veneered only by 
coral limestone over its very top. The eastern and 
western edges of Florida consist of recent limestones, 
predecessors of that now forming on the western and 
southern slopes of the Florida plateau. Very probably 
the part of the peninsula north of the Everglades has 
had in the past a like formation. Pourtales plateau is 
built of the same species of corals and shells as now live 
